Conventional aircraft are known which comprise a plurality of propellers mounted on wings, which are in turn attached to a fuselage. Control of the aircraft is provided by control surfaces such as ailerons, elevators, and rudders, which can be differentially deflected to effect roll, pitch and yaw respectively, or with a combination of all three. High lift devices such as flaps and slats may also be provided, which can be selectively deployed to increase the lift produced by the wing at the expense of increased drag.
One of the defining features of a wing is the “aspect ratio” which can be defined as the ratio of the square of the span of the wing to its area. In general, a high aspect ratio wing will produce less lift induced drag for a given amount of lift and so, a higher aspect ratio wing will have a greater aerodynamic efficiency compared to a low aspect ratio wing, and so will contribute to a lower fuel burn.
In practice however, very high aspect ratio wings (greater than around 10 to 15) are difficult to achieve in view of structural considerations. As a result of their relatively long length and narrow chord, high aspect ratio wings generally have a lower stiffness (both in terms of torsion about the span of the wing and bending movements along the span of the wing) compared to low aspect ratio wings for given construction methods and materials. Consequently, in order to obtain a high aspect ratio wing, either expensive construction methods and materials must be used, or an increased structural weight must be incurred, thereby reducing the fuel burn benefits of the high aspect ratio wing in view of the greater amount of lift (and therefore drag) that must be produced by the wing to accommodate the increased weight.
High aspect ratio wings also suffer from wind gusts in flight, due to their large increase in lift with increasing angle of attack (a). This may preclude high aspect ratio wings on civil aircraft in view of passenger comfort and/or airframe fatigue considerations, particularly if these aircraft operate at low altitudes for a significant length of time.
In a separate problem, moveable control surfaces such as flaps, ailerons, elevators and rudders are generally aerodynamically inefficient in view of the additional drag created by the control surface when deflected, and add weight to the aircraft. In general, ailerons also affect the lift distribution on the wing, taking the distribution away from the ideal elliptical distribution, and so increasing lift induced drag. Similarly, flaps and slats are relatively aerodynamically inefficient particularly in view of the strong tip vortices shed by such devices, and the effect they have on lift distribution. They are also heavy and complex, particularly in view of the requirement for hydraulic or electric actuators for deploying them.
In a further separate problem, it may be desirable for an aircraft to take off and land in a short distance (i.e. Short Takeoff and Landing, STOVL), or even vertically (i.e. Vertical Take Off and Landing, VTOL). One proposal is to mount the wings to the aircraft on a pivot. When a VTOL takeoff or landing is performed, the wings are pivoted to an upward position, with both the wings and propellers facing upwards. The wings are pivoted to a horizontal position for forward flight. One known example is the GL-10 Greased Lightning, described in “NASA Langley Distributed Propulsion VTOL TiltWing Aircraft Testing, Modeling, Simulation, Control, and Flight Test Development” by Rothhaar et al, published in 14th AIAA Aviation Technology, Integration and Operations Conference. However, previous proposed aircraft have included relatively heavy, complex mechanisms for tilting the wing. It is therefore desirable to provide a lightweight, reliable mechanism for converting a tiltwing aircraft between vertical and horizontal flight configurations.
The present invention describes an aircraft control arrangement and a method of controlling an aircraft which seeks to overcome some or all of the above problems.